1. Field of the Invention
The present invention relates to a ball screw mechanism and a steering device.
2. Description of the Related Art
Hitherto, there has been a steering device that applies an assist force to a steering mechanism, in which rotation of a motor is converted into reciprocal motion of a rack shaft using a ball screw mechanism to apply the rotation from the motor as the assist force. International Publication No. WO 2007/114036 describes an example of such a ball screw mechanism in which a screw shaft and a nut are threadably engaged with each other via a multiplicity of balls.
In the ball screw mechanism according to WO 2007/114036, a rolling groove for the balls is spirally formed on the outer peripheral surface of the screw shaft. In addition, a load rolling groove for the balls that faces the rolling groove for the balls in the screw shaft is spirally formed on the inner peripheral surface of the nut. Meanwhile, a no-load rolling groove for the balls is spirally formed along the load rolling groove to be deeper than the load rolling groove. Then, in the ball screw mechanism, a direction changing groove is further formed on the inner peripheral surface of the nut so that respective ends of the load rolling groove and the no-load rolling groove are coupled to each other on the inner peripheral surface of the nut. This eliminates the need for a so-called deflector (a component that forms a circulation path for the balls) for endless circulation of the balls.
In the ball screw mechanism according to WO 2007/114036, however, the no-load rolling groove and the direction changing groove are formed along the load rolling groove on the inner peripheral surface of the nut. Thus, a dimensional margin for forming the no-load rolling groove between load rolling grooves and a dimensional margin for forming the direction changing groove must be secured, which inevitably increases the size of the nut, that is, the ball screw mechanism.